The long-term aims of the proposed experiments are to elucidate the circuitry responsible for the functional organization of trigeminal (V) second-order neurons, and to determine how their outputs are distributed and functionally reflected in V third-order target neurons. Five studies are proposed: 1) Anterograde bulk-transport and immunocytochemical techniques will be used to visualize the distribution and structure of higher-order projections (serotonergic, adrenergic, GABAergic, intersubnuclear, cortical) to each of the V brainstem subnuclei. Higher-order and primary afferent "contacts" onto functionally characterized and peroxidase- stained cells will also be assessed at light and electron microscopic levels. 2) To determine the functions of higher-order inputs, single unit recording, electrical stimulation and receptive field mapping will be applied to V second-order cells subsequent to either of the following manipulations: electrical stimulation of somatosensory cortex or raphe nuclei, ablation of somatosensory cortex, ablation of intersubnuclear-projecting V brainstem cells with a cell specific neurotoxin, pharmacological depletion of serotonin or norepinephrine, or pressure microejection of the latter or their antagonists. 3) To determine whether V brainstem cells have presynaptic GABA-containing processes which gate transmission within the V brainstem complex, the ultra-structure of physiologically identified local circuit neurons. 4) To test the hypothesis that functionally distinct V-thalamic cells have anatomically distinct axon terminal arbors, a plant lectin (PHA-L) will be iontophoresed into identified V-thalamic cells which will immuno-cytochemically label their terminals. 5) Convergence of principalis and spinal V inputs onto third-order V neurons will be assessed anatomically and physiologically. Orthograde bulk-tracing procedures will be used to map the spatial organization of V brainstem terminals in thalamus. Thalamic cell responses will then be correlated with the extent to which electrical stimulation of different V subnuclei orthodromically activate the cell. These studies are unusual in combining a wide range of techniques to provide simultaneous morphological, physiological, pharmacological, and connectivity data for individual V neurons, their inputs and outputs. Thus, circuits which collectively synthesize V brainstem function will be uncovered. These studies will also provide a normative baseline for future studies of central V reorganization induced by craniofacial injury.